Dumping iron in the ocean may not fix the climate

As arguments rage over the legality of an attempt to fertilise algae in Antarctic seas, evidence is emerging that, legal or not, this kind of "ocean engineering" may not suck enough carbon out of the atmosphere to reverse climate change.

A new study confirms that iron-enriched waters do, as hoped, encourage more carbon to be stored on the ocean floor. But the efficiency of artificial iron fertilisation could be as much as 50 times lower than previous estimates.

In some oceans, the Southern Ocean in particular, phytoplankton growth is limited by the amount of iron in the water. The hope is that by dumping iron into the water, we could stimulate plankton blooms that will absorb carbon dioxide from the atmosphere. When the plankton die and drop to the ocean floor, they should drag carbon down with them.

The question is, how much carbon gets dragged down and stored on the sea floor and how much is recycled higher up in the water column?

Natural experiments

Plankton bloom naturally every year north of Crozet, boosted by the iron swept off the island's volcanic flanks. Pollard and colleagues monitored the fallout from one of these blooms 100 metres and 300 metres beneath the surface, and compared this with the carbon fallout in an area that did not bloom.

They found that three times as much carbon falls to the sea floor beneath a bloom as in "clear" waters. Sediment cores taken from the sea floor suggest the carbon stays there for thousands of years.

"Our carbon-to-iron ratio is 80 times less than one reported by another study a year ago," says Pollard. He believes the previous experiment underestimated the amount of iron that was seeping into the bloom area.

He says the more optimistic figure "was so high it might have given geoengineers great hope that they could stick in not much iron and get huge quantities of carbon sequestered".

Limiting factors

Pollard accepts that artificial schemes might be more effective than natural blooms at boosting carbon stores. Suddenly dumping large amounts of iron overboard is quite different from the slow flow of iron into the ocean through natural processes, and it is possible that this would prompt different species of plankton to bloom, sequestering more carbon.

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Plankton blooms - can seeding them with iron cool the planet? (Image: European Space Agency)